1. Field of the Invention
The present invention relates to a power source circuit which is capable of monitoring a voltage of a secondary battery to control charge and discharge.
2. Description of the Related Art
FIG. 3 is a block diagram, partly in circuit diagram, showing schematically a configuration of a conventional power source circuit.
A negative external terminal 313 to which either a battery charger 310 or a load 311 is to be connected, is connected to a negative electrode of a secondary battery 301. A positive external terminal 312 to which the battery charger 310 or the load 311 is to be connected, is connected to a positive electrode of the secondary battery 301 through a switching device (P-channel MOS FET) 302 and a switching device (P-channel MOS FET) 303 connected in series. In addition, a charge and discharge controlling circuit 316 is connected in parallel with the secondary battery 301. The charge and discharge controlling circuit 316 has the function of detecting a voltage of the second battery 301 and a voltage developed across the switching devices 302 and 303.
That is, the charge and discharge controlling circuit 316 has the function of detecting the state in which the battery voltage of the secondary battery 301 is lower than a predetermined voltage value to turn OFF the switching device 302. Then, this state is referred to as xe2x80x9cthe over-discharge detection statexe2x80x9d when applicable. In addition, the circuit 316 has the function of detecting the state in which the voltage developed across a VDD terminal 314 and a terminal 309 for detecting the battery charger connection and the over-current, is larger than a predetermined voltage value to turn OFF the switching device 302. Then, this state is referred to as xe2x80x9cthe over-current detection statexe2x80x9d when applicable. Also, the circuit 316 has the function of detecting the state in which the battery voltage of the secondary battery 301 is higher than a predetermined voltage value to turn OFF the switching device 303. Then, this state is referred to as xe2x80x9cthe over-charge detection statexe2x80x9d when applicable.
Now, the description will hereinbelow be given with respect to the operation in the case of the over-charge state. The battery charger 310 is connected between the positive external terminal 312 and the negative external terminal 313 to charge the secondary battery 301 with the electric charges, thereby providing the state in which the voltage developed across the VDD terminal 314 and a VSS terminal 315 is higher than a predetermined voltage value. At the time when this state has been provided, an over-charge and over-discharge detecting circuit 317 sends an over-charge detection signal to a logic circuit 318 for controlling switching devices. In response to the over-charge detection signal, the logic circuit 318 for controlling switching devices sends a signal to turn OFF the switching device 302 through a charge control terminal 304.
Next, the description will hereinbelow be given with respect to the operation in the case of the over-discharge state. The load 311 is connected between the positive external terminal 312 and the negative external terminal 313 to discharge the secondary battery 301, thereby providing the state in which the voltage developed across the VDD terminal 314 and the VSS terminal 315 is lower than the predetermined voltage value. At the time when this state has been provided, the over-charge and over-discharge detecting circuit 317 sends an over-discharge detection signal to the logic circuit 318 for controlling switching devices. In response to the over-discharge detection signal, the logic circuit 318 for controlling switching devices sends a signal to turn OFF the switching device 303 through a discharge control terminal 305.
Finally, the description will hereinbelow be given with respect to the operation in the case of the over-current state. At the time when the terminal voltage developed across the VDD terminal 314 and the terminal 309 for detecting the battery charger connection and the over-current, has become larger than a predetermined voltage value, a circuit 319 for detecting the battery charger connection and the over-current sends an over-current detection signal to the logic circuit 318 for controlling switching devices. In response to the over-current detection signal, the logic circuit 318 for controlling switching devices sends a signal to turn OFF the switching device 303 through the discharge control terminal 305. In addition, when the load 311 is connected to provide the over-current state, the terminal 309 for detecting the battery charger connection and the over-current is pulled down. However, at the time when the load 311 between the positive external terminal 312 and the negative external terminal 313 is released, a signal to turn ON the P-channel MOS FET 306 is sent from the logic circuit 318 for controlling switching devices to pull up the terminal 309 for detecting the battery charger connection and the over-current to VDD, thereby carrying out the release of the over-current state.
Here, when the battery charger 310 is connected in the over-charge detection state, even if the switching device 303 is turned OFF, the current is caused to flow from the battery charger 310 to the secondary battery 301 through a current path 320 passing through the positive external terminal 312, the terminal 309 for detecting the battery charger connection and the over-current, and a parasitic diode 308 of the P-channel MOS FET 306 so that the battery 301 is charged with the electric charges. For this reason, a resistor 307 is provided in order to limit an amount of current which is caused to flow from the battery charger 310 to the secondary battery 301.
As described above, in the conventional power source circuit, the resistor 307 is provided in order to limit an amount of current which is caused to flow from the battery charger 310 to the secondary battery 301. However, since the current is caused to flow through the resistor, it is impossible to cut off perfectly the current path.
In the light of the foregoing, the present invention has been made in order to solve the above-mentioned problems associated with the prior art, and it is therefore an object of the present invention to prevent a current from being caused to flow into a secondary battery by providing a diode instead of employing a resistor having a fixed magnitude of resistance.
Another object of the present invention is to provide a power source circuit having a current limit function of adjusting an amount of current of a secondary battery, the power source circuit comprising: a negative external terminal through which one of a battery charger and a load is connected to a negative electrode side of the secondary battery; a positive external terminal through which one of the battery charger and the load is connected to a positive electrode side of the secondary battery via a switching device connected in series with the positive electrode side of the secondary battery; and a charge and discharge controlling circuit connected in parallel with the secondary battery, wherein the charge and discharge controlling circuit includes: a voltage detecting circuit for detecting a voltage of the secondary battery to output a signal; a switching-device-controlling logic circuit for in response to the signal from the voltage detecting circuit, controlling the switching device; a current detecting circuit for outputting a signal to control the switching-device-controlling logic circuit on the basis of a value of the electric potential difference between a voltage of the positive electrode of the secondary battery and a voltage of the positive external terminal; a transistor having a gate electrode to which a signal having the information that exhibits the over-current state is inputted from the switching-device-controlling logic circuit; and a diode connected between the transistor and the current detecting circuit for cutting off a current from a parasitic diode of the transistor.